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No description available.

No description available.

The flux of carbon in to and out of the atmosphere exerts a fundamental control over the Earth's climate. The oceans contain almost two orders of magnitude more carbon than the atmosphere, and consequently, small fluctuations within the oceanic carbon reservoir can have very significant effects on air-sea CO₂ exchange, and the climate of the planet. Pelagic carbonates represent a major long-term flux of carbon from the surface ocean to deep-sea sediments. Within sediments, the biologically produced carbonates act as a longterm carbon store, but also as chemical recorders of past surface ocean conditions. Counterintuitively, despite the production and sedimentation of carbonate acting as a CO₂ sink, over periods shorter than the mixing-time of the ocean, the pH change associated with calcium carbonate precipitation enriches the surface waters in CO₂ and elevates the equilibrium value of gaseous exchange with the atmosphere. Coccolithophores, ubiquitous marine photosynthetic plankton, produce calcium carbonate plates, coccoliths, which account for around one third of all marine calcium carbonate production. Sedimentary coccoliths therefore represent a valuable repository of surface ocean geochemical data, as well as a very significant carbon-cycle flux. This thesis examines how the mass of calcium carbonate produced by coccolithophores has changed in response to rising levels of atmospheric CO₂. A -40% increase in average coccolith mass over the last 230 years, paralleling anthropogenic CO₂ release, is demonstrated within a high-accumulation rate North Atlantic sediment core. Additionally, a flow-cytometry method is presented, which enables the automatic separation of coccoliths from clay particles in sedimentary samples, representing the first step in a coccolith cleaning procedure, which should ultimately enable down-core measurements of coccolith trace-element/calcium ratios. Complementing this work I describe results from continuous dissolution analysis of cultured coccoliths which allows a first-order evaluation of trace-element partitioning into coccoliths produced by the species Coccoliths pelagicus, and present a conceptual methodology to allow the determination of single-species coccolith chemical data.

The end-Permian extinction at 252 Ma is widely regarded as the most severe of the Phanerozoic mass-extinctions and enabled the evolution of the modern carbon cycle and ecosystem structure. The cause of the extinction is still debated but the synergistic pressures of global climate change, such as anoxia and ocean acidification, were clearly important. The extinction occurred in two phases and is marked by a uniquely protracted recovery period of ~ 5 Myrs where diversity fails to reach pre-extinction levels until the Middle Triassic. This period is characterized by an unstable global carbon cycle, secondary extinctions, reef, chert and coal gaps, and changes in the carbonate factory from reef to microbial and abiotic dominated deposition. This thesis focuses on using geochemical data from the Arabian Margin to investigate the carbon cycle record and the links between kill mechanisms and carbon cycle dynamics. A new record of carbon cycling is presented for the Tethys in the form of a carbon isotope record for the entire Early Triassic from the Musandam Peninsula, United Arab Emirates (UAE). The Musandam carbon isotope record can be broadly correlated with global isotopic events but also resolves additional secondary excursions. These new short-lived events are probably related to the occurrence of the more widely recognized Early Triassic excursions, and may represent fluctuations in the driving mechanisms superimposed on the continued instability of the global carbon cycle in the aftermath of the end-Permian extinction. To unravel palaeo-depositional redox conditions this work utilizes geochemical proxies based on Fe systematics (Fe-speciation). To date, however, these proxies have only been calibrated in relation to modern and ancient siliciclastic marine sediments. This clearly limits the use of the Fe-speciation proxy, particularly in relation to carbonate-rich sediments and rocks. This thesis explores the use of Fe-speciation in carbonates using compiled literature and new data from modern oxic and anoxic settings. This new assessment expands the utility of Fe-based redox proxies to also incorporate carbonate-rich rocks that contain significant total Fe (>0.5 wt%), providing care is taken to assess possible impacts of diagenetic processes such as dolomitization. Based on this calibration work Fe-speciation is used to reconstruct the redox structure for the Arabian Margin mixed carbonate and clastic sediments, from the late Permian to the Middle Triassic. Fe-S-C systematics are utilized to identify the spatial and temporal dynamics of anoxia for a Neo-Tethyan shelf-to-basin transect. The unique spatial resolution afforded by this transect allows a direct link to be drawn between biodiversity, carbon cycling and anoxic events. For the first time we can directly observe a switch from deep-ocean dominated anoxia to a dynamic anoxic wedge at the end-Permian extinction. Additionally the data suggest that ferruginous conditions (anoxic non-sulphidic) were dominant in the Tethyan Ocean throughout the Early Triassic, proposing that euxinia was restricted regionally with potential implications for nutrient recycling, carbon cycle models and driving mechanisms. Redox dynamics may have had important implications for the wider carbonate cycle. These two themes are particularly inter-related with regards to oceanic alkalinity and pH. This thesis presents the first shallow water boron isotope record for the Permian Triasssic Boundary, used as a proxy for pH. The record demonstrates some unexpected results; firstly a sudden increase in pH is observed, prior to the first phase of the extinction and interpreted to reflect alkalinity supply from the development of slope anoxia. Secondly there is no evidence for an acidification event at the first phase of the extinction where pH remains stable. A rapid acidification event is, however, seen in the earliest Triassic, contemporary to the second phase of the mass extinction, but delayed compared to the main negative carbon isotope excursion that indicates the main phase of Siberian Trap volcanism. These events may be explained by dramatic changes in ocean the ocean’s buffering capacity linked to changes in alkalinity supply and the carbonate factory.

Graduation date: 2002

The purpose of this thesis is to provide a first look at the spatial and temporal distributions of dissolved organic material (DOM) off the Oregon coast of North America. While this paper is not a comprehensive examination of these distributions, several patterns are identified as promising candidates for continued research. Most of the data presented was acquired during a strong El Nino event. The DOM data is presented as dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) and is accompanied by temperature, salinity, nitrate plus nitrite (N+N), ammonium, silicate, chlorophyll, total organic carbon (TOC), particulate organic carbon (POC), total nitrogen (TN), total organic nitrogen (TON), and zooplankton biomass measurements. During July 1997, we examined the distribution of DOM in the surface waters off the Oregon and Southern Washington coasts. Eleven east-west transects were sampled from nearshore waters to 190km offshore. DOC concentrations as high as 180 iM were observed in the Columbia River plume. Patterns in the DOC distribution were also associated with upwelling regions, an offshore coastal jet, and an oligotrophic water mass in the northern portion of our study area. Beginning with the July 1997 study and continuing until July 1998, samples were collected on weekly and seasonal time scales at station NH-05, located 9km offshore from Newport, Oregon. Various problems have limited our seasonal comparisons, but we were able to collect high quality data depicting the changes in organic matter partitioning during a phytoplankton bloom and its decline during a two month period from mid-July through mid-September in 1997. During the bloom, POC increased dramatically, but DOC decreased. Possible explanations for this decrease and for changes in the C/N ratio of the DOM during the bloom are explored. Suggestions for future research are presented in the final chapter. / Graduation date: 2000

Graduation date: 2003

Thesis (MSc)--University of Stellenbosch, 2005. / Bibliography. Also available via the Internet.

Thesis (Ph.D.)--University of Delaware, 2006. / Principal faculty advisor: David L. Kirchman, College of Marine and Earth Studies. Includes bibliographical references.